Magnetic recording medium

ABSTRACT

The present invention provides a magnetic recording medium which can record both digital and analog signals by, e.g., VTR for commercial use and improve their electromagnetic conversion characteristics. A magnetic recording medium comprises a nonmagnetic support, and first and second magnetic layers comprised respectively of magnetic powder (A) and (B), wherein the first and second magnetic layers are formed on the nonmagnetic support in this order, wherein the magnetic powder (A) and (B) have the following characteristics:
         saturation magnetization σs(A): 80 to 150 Am 2 /kg;   coercive force Hc(A): 90 to 180 kA/m,   saturation magnetization σs(B): 100 to 200 Am 2 /kg,   coercive force Hc(B): 100 to 200 kA/m,   wherein the first and second magnetic layers satisfy the below relationships and have thickness T(A) and T(B) in the below ranges:
 
σ s ( A )/σ s ( B )≦1.0, Hc ( A )/ Hc ( B )≦1.0,
 
1.0 μm≦ T ( A )≦4.0 μm, 0.01 μm≦ T ( B )≦0.5 μm.

The subject matter of application Ser. No. 10/422.720, is incorporatedherein by reference. The persent application is a continuation of U.S.application Ser. No. 10/442,720, filed May 21, 2003, now U.S. Pat. No.6,929,850, which claims priority to Japanese Patent Application No.JP2002-173544, filed Jun. 14, 2002, and Japanese Patent Application No.JP2003-005227, filed Jan. 14, 2003. The present application claimspriority to these previously filed applications.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic recording medium.

2. Description of Related Art

Recently, digital broadcasting and data broadcasting are being spread inthe television broadcasting filed, and, as magnetic recording media foraudio and video signals, recording media products which can record orreproduce digital data are becoming a mainstream.

Now, it is a period of transition from analog era to digital era, andtherefore a video tape recorder (VTR) or a data tape drive which canreproduce and edit both the old analog data and the new digital data isconvenient, and VTR and data tape drive systems that can be used foranalog and digital data are available.

Many of currently available digital VTRs and data tape drives forcommercial use or for broadcasting use are designed to use a format torecord digital signals and time code, i.e., time signals as well asanalog audio signals simultaneously.

However, there is a difference in the frequency and depth of therecorded signal between the digital signals and the analog audiosignals. Therefore, it is difficult to secure excellent properties ofall the signals, and a number of vigorous studies are being made on themagnetic recording media.

For example, there is a method in which a magnetic coating compositioncomprising finely divided magnetic powder, which can achieve digitalrecording at shorter wavelengths, is applied so that the resultantmagnetic layer has a thickness of 2 to 5 μm which corresponds to thedepth used for analog recording.

However, this method has a problem in that the too large thickness ofthe magnetic layer lowers the output in electromagnetic conversioncharacteristics, that is, a problem of self-demagnetization due to thelarge thickness.

As a method for solving the above-mentioned problem, a technique isgenerally used in which a thick nonmagnetic layer is formed as a lowerlayer on the surface of the nonmagnetic support, and a magnetic layer isformed as an upper layer on the nonmagnetic layer so that the thicknessof the magnetic layer is reduced to suppress demagnetization due to thelarge thickness, achieving a larger output. However, this method isunsuitable for analog signals since the lower layer is nonmagnetic.

On the other hand, there is a Mag-on-Mag method in which a magneticlayer suitable for analog recording is formed as a lower layer, andanother magnetic layer suitable for digital recording is applied ontothe lower layer to enable both analog recording and digital recording(see, for example, the below-listed Patent document 1 or 2). Thistechnique of forming the above multi-layered magnetic layer has afeature such that the magnetic recording medium is designed so that theanalog audio signals having longer wavelengths that need a recordingfrequency as low as about 1 KHz are recorded on the magnetic recordingmedium in a depth of 2 to 3 μm, and video signals having shorterwavelengths that need a recording frequency as high as several toseveral tens MHz are recorded on the magnetic recording medium at thesurface layer in a depth of 0.3 μm or less.

Specifically, this is a method in which the upper layer is formed frommagnetic powder of fine particle suitable for the higher recordingfrequency, the lower layer is formed from another magnetic powder havingmagnetic properties such that a satisfactory output can be obtained evenin a longer wavelength region, and the two types of magnetic layers arestacked on one another.

See [Patent document 1]: Unexamined Japanese Patent ApplicationLaid-Open Specification No. 53-54002 (page 2, right-hand upper column,line 10 to left-hand lower column, line 1, and FIG. 1).

See [Patent document 1]: Unexamined Japanese Patent ApplicationLaid-Open Specification No. 3-62315 (page 2, right-hand upper column,line 8 to right-hand lower column, line 20, and FIG. 1).

Various types of Mag-on-Mag methods have conventionally been studied,and a number of studies have been made on VHS and the like in a formatsuch that video signals are overwritten on audio signals, but, due tothe above-mentioned problem of self demagnetization, for bringing themagnetic recording media produced by this method into practical use,there are restrictions of the sizes and magnetic properties of themagnetic materials used for the upper and lower layers and the thicknessand construction of the individual layers.

SUMMARY OF THE INVENTION

A task of the present invention is to provide a magnetic recordingmedium which can record both digital signals and analog signals by,e.g., VTR for commercial use or for broadcasting use, and which canimprove their electromagnetic conversion characteristics.

Specifically, in the present invention, there is provided a magneticrecording medium which comprises a nonmagnetic support, a first magneticlayer comprised of magnetic powder (A), and a second magnetic layercomprised of magnetic powder (B), wherein the first and second magneticlayers are formed on the nonmagnetic support in this order, wherein themagnetic powder (A) used in the first magnetic layer has the followingcharacteristics:

-   -   a saturation magnetization σs(A) of 80 to 150 Am²/kg; and a        coercive force Hc(A) of 90 to 180 kA/m,    -   wherein the magnetic powder (B) used in the second magnetic        layer has the following characteristics:    -   a saturation magnetization σs(B) of 100 to 200 Am²/kg; and a        coercive force Hc(B) of 100 to 200 kA/m,        wherein the first and second magnetic layers satisfy the        following relationships:        σs(A)/σs(B)≦1.0, and Hc(A)/Hc(B)≦1.0,    -   wherein the first magnetic layer has a thickness T(A) in the        range represented by the following formula:        1.0 μm≦T(A)≦4.0 μm,    -   and wherein the second magnetic layer has a thickness T(B) in        the range represented by the following formula:        0.01 μm≦T(B)≦0.5 μm.

According to the present invention, the ranges of the saturationmagnetization σs(A) and coercive force Hc(A) of the magnetic powder (A)used in the first magnetic layer (lower layer) and the ranges of thesaturation magnetization σs(B) and coercive force Hc(B) of the magneticpowder (B) used in the second magnetic layer (upper layer) areindividually defined as mentioned above. Therefore, not only be theformat of digital recording met, but also the properties of deep layerrecording for analog audio can be simultaneously improved. Particularly,close studies have been made on the properties of the magnetic powderused in each of the first magnetic layer (lower layer) and the secondmagnetic layer (upper layer). As a result, it has been found that, whenthe ratio of the thickness of the second magnetic layer (upper layer) tothe thickness of the first magnetic layer (lower layer) is extremelyreduced as mentioned above, the resultant magnetic recording medium canrecord both digital signals and analog signals and improve theirelectromagnetic conversion characteristics.

Specifically, there can be realized a magnetic recording medium whichcan record both digital signals and analog signals by, e.g., VTR forcommercial use or for broadcasting use, and which can improve theirelectromagnetic conversion characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description ofthe presently preferred exemplary embodiments of the invention taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a diagrammatic cross-sectional view of a magnetic recordingmedium according to one embodiment of the present invention;

FIG. 2 is a graph showing the range of the saturation magnetization σsof the magnetic powder used in the magnetic layer in the magneticrecording medium;

FIG. 3 is a graph showing the range of the coercive force Hc of themagnetic powder used in the magnetic layer in the magnetic recordingmedium;

FIG. 4 is a graph showing the electromagnetic conversion characteristicsin the Example of the present invention when the thickness of the upperlayer was fixed at 0.3 μm and the thickness of the lower layer waschanged;

FIG. 5 is a graph showing the electromagnetic conversion characteristicsin the Example of the present invention when the thickness of the lowerlayer was fixed at 2.7 μm and the thickness of the upper layer waschanged;

FIG. 6 is a graph showing the electromagnetic conversion characteristicsin the Example of the present invention when the values other than thesaturation magnetization σs(A) of the lower layer were fixed and theσs(A) was changed;

FIG. 7 is a graph showing the electromagnetic conversion characteristicsin the Example of the present invention when the values other than thecoercive force Hc(A) of the lower layer were fixed and the Hc(A) waschanged;

FIG. 8 is a graph showing the electromagnetic conversion characteristicsin the Example of the present invention when the values other than thesaturation magnetization σs(B) of the upper layer were fixed and theσs(B) was changed;

FIG. 9 is a graph showing the electromagnetic conversion characteristicsin the Example of the present invention when the values other than thecoercive force Hc(B) of the upper layer were fixed and the Hc(B) waschanged;

FIG. 10 is a graph showing the electromagnetic conversioncharacteristics in the Example of the present invention when theσs(A)/σs(B) ratio was changed;

FIG. 11 is a graph showing the electromagnetic conversioncharacteristics in the Example of the present invention when theσs(A)/σs(B) ratio was changed;

FIG. 12 is a graph showing the electromagnetic conversioncharacteristics in the Example of the present invention when theHc(A)/Hc(B) ratio was changed;

FIG. 13 is a graph showing the electromagnetic conversioncharacteristics in the Example of the present invention when theHc(A)/Hc(B) ratio was changed;

FIG. 14 is a graph showing the electromagnetic conversioncharacteristics in the Example of the present invention when theHc(A)/Hc(B) ratio was changed by changing the Hc(A) while fixing theHc(B); and

FIGS. 15 to 44 are tables (Tables 1 to 30) showing the formulations ofthe upper layer magnetic powder and the lower layer magnetic powder usedin the cassette tapes prepared in the Examples and Comparative Examples,and the results of the measurement of properties of the cassette tapes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in the diagrammatic cross-sectional view of FIG. 1 of amagnetic recording medium of the present invention, a magnetic recordingmedium 1 of the present invention comprises a nonmagnetic support 2, afirst magnetic layer (lower layer) 3 a comprised of magnetic powder, anda second magnetic layer (upper layer) 3 b comprised of magnetic powder,wherein the first and second magnetic layers are formed on thenonmagnetic support in this order. If desired, a back coat layer 4 maybe formed on another surface of the nonmagnetic support 2 that is notthe surface on which the magnetic layers 3 are formed.

With respect to the range of saturation magnetization σs:

-   -   In the magnetic recording medium of the present invention, the        ferromagnetic fine powder usable in each of the first magnetic        layer (lower layer) and the second magnetic layer (upper layer)        is acicular Fe (so-called metal magnetic powder). Specifically,        the metal magnetic powder having a saturation magnetization σs        which falls within the above-mentioned range, namely, the shaded        area shown in FIG. 2 is appropriately selected and used. The        saturation magnetization σs indicates the magnetic energy        inherent in the individual magnetic powder.

When the saturation magnetization σs(A) of the magnetic powder used inthe first magnetic layer (lower layer) exceeds 150 Am²/kg, the effect ofdemagnetization is increased. Further, for securing excellentelectromagnetic conversion characteristics of analog signals, it isnecessary that the σs(A) be 80 Am²/kg or more and the thickness of thefirst magnetic layer be 1.0 to 4.0 μm.

For recording digital signals with higher density, the ferromagneticfine powder usable in the second magnetic layer (upper layer) needs ahigher saturation magnetization σs(B). For example, in video tapes in aformat of, e.g., 8 mm currently used, ED-beta or beta-cam SP, or digitalbeta-cam, the ferromagnetic powder having a σs(B) of 100 to 150 Am²/kgis effective. In high capacity data storage DTF-2 and the like, theferromagnetic powder having a σs(B) of 150 to 200 Am²/kg is effective.The metal magnetic powder having a σs(B) of higher than 200 Am²/kg hastoo high cohesion and hence has poor dispersibility, and therefore thismetal magnetic powder is not suitable for coating the second magneticlayer (upper layer) in a thin film form.

It is desired that the saturation magnetization σs(A) of the firstmagnetic layer is lower than the saturation magnetization σs(B) of thesecond magnetic layer. When the σs(A) is higher than the σs(B), a lossdue to the above-mentioned self demagnetization (deterioration of themagnetic properties and electromagnetic conversion characteristics ofthe magnetic recording medium) is likely to occur.

With respect to the range of coercive force Hc:

In the magnetic recording medium of the present invention, as theferromagnetic fine powder usable in each of the first magnetic layer(lower layer) and the second magnetic layer (upper layer), the metalmagnetic powder having a coercive force Hc which falls within theabove-mentioned range, namely, the shaded area shown in FIG. 3 isappropriately selected and used.

When the coercive force Hc(A) of the first magnetic layer (lower layer)falls outside of the above range, the electromagnetic conversioncharacteristics of analog signals deteriorate similarly when the σsfalls outside of the above corresponding range. Further, in the magneticrecording medium of the present invention, for example, Fe-based metalmagnetic powder is used in both the upper and lower layers, andtherefore the lower limit of the Hc(A) of the lower layer is about 90kA/m.

On the other hand, the upper limit of the coercive force Hc(B) can beincreased by, for example, increasing the Co content of the magneticpowder, but, taking into consideration of the format of the VTR or datatape drive, when the Hc(B) is higher than 200 kA/m, the recordingcurrent is not appropriate, causing a problem that the erase propertiesare poor.

With respect to the analog audio frequency characteristic, when thecoercive force Hc(A) of the lower layer is lower, the output at afrequency as low as 1 kHz is considerably increased and the output at afrequency as high as 12 kHz tends to be lowered. Further, the analogaudio output depends on the residual magnetic flux density (Br) of thelower layer, and therefore, when magnetic powder having larger magneticenergy and higher saturation magnetization σs is used, the output islikely to be affected by the residual magnetic flux density.

The present inventors have made studies closely on this phenomenon. As aresult, they have found for the first time that this phenomenon isremarkable when the difference in the Hc between the upper layer and thelower layer is too large. That is, they have found that, when thedifference in the Hc between the upper layer and the lower layer issmaller, the analog audio frequency characteristic can be effectivelyflatten.

With respect to the more advantageous range of the coercive force Hc,the upper layer and the lower layer satisfy the following relationship:0.8≦Hc(A)/Hc(B)≦1.0,more preferably,0.85≦Hc(A)/Hc(B)≦1.0.

For obtaining excellent sensitivity (output at 1 kHz) in the analogsignal properties, it is effective that the Hc(A) of the lower layer is90% or less of the Hc(B) of the upper layer.

With respect to the Co content:

-   -   As mentioned above, the ferromagnetic fine powder usable in the        magnetic recording medium of the present invention is acicular        metal magnetic powder, and preferably contains cobalt (Co) so as        to meet a predetermined requirement.

The Co contained in the magnetic powder in the second magnetic layer(upper layer) contributes to improvement of the Hc(B) and σs(B) andreduction in the crystallite size, and further it effectively avoidslowering of Δσs, which indicates the storage stability of the magneticrecording medium, but, when the Co content of the magnetic powder isless than 3 at. %, the above effects cannot be obtained satisfactorily.On the other hand, when the Co content of the magnetic powder exceeds 50at. %, the coercive force Hc(B) is lowered. Therefore, the Co content ofthe magnetic powder in the upper layer may be 3 to 50 at. %, morepreferably 5 to 40 at. %, further preferably 5 to 35 at. %. The unit“at. %” means an atom percentage.

On the other hand, it is preferred that the Co content of the magneticpowder in the first magnetic layer (lower layer) is appropriatelyselected depending on the Hc(A) and σs(A), preferably 0 to 5 at. %,since Co is expensive.

As other elements added to the magnetic powder in each of the firstmagnetic layer and the second magnetic layer, ones conventionally knowncan be used.

For example, aluminum (Al) has a remarkable effect to improve theacicular finely divided powder in dispersibility (sinter preventingproperty) and in retention of particle form during reduction. When theAl content of the magnetic powder is less than 0.1 at. %, it isdifficult to obtain the above effect. On the other hand, when the Alcontent of the magnetic powder exceeds 20 at. %, the saturationmagnetization σs of the magnetic layer is lowered, so that the magneticproperties deteriorate. Therefore, the Al content of the magnetic powdermay be in the range of 0.1 to 20 at. %, preferably 1 to 15 at. %,further preferably 5 to 10 at. %. When Al is contained in the magneticpowder in the form of a compound (oxide), the Al content of the magneticpowder does not mean a content of the Al compound in the magnetic powderbut a content of the Al element-contained in the Al compound in themagnetic powder.

Further, like Al, rare earth elements {including yttrium (Y)}effectively improve the metal powder in sinter preventing property anddispersibility. When the rare earth element content of the magneticpowder is less than 0.1 at. %, the effect of the rare earth element istoo small and hence the magnetic powder is likely to be sintered. On theother hand, when the rare earth element content of the magnetic powderexceeds 10 at. %, the amount of the oxide of the element is increasedand hence the saturation magnetization σs of the magnetic layer becomestoo small, so that the magnetic powder is inappropriate as the metalmagnetic powder for the second magnetic layer (upper layer). Examples ofrare earth elements include Y, La, Ce, Pr, Nd, Sm, Tb, Dy, and Gd, and,when these rare earth elements are used in combination, the content ofthe sum of the elements in the magnetic powder may be 0.1 to 10 at. %.When the rare earth element is contained in the magnetic powder in theform of a compound, the rare earth element content of the magneticpowder does not mean a content of the compound in the magnetic powderbut a content of the rare earth element contained in the compound in themagnetic powder.

Further, in the magnetic recording medium of the present invention, withrespect to the particle size of the metal powder used in each of theupper and lower layers, the average major axis length is preferably 0.01to 0.5 μm, more preferably 0.4 to 0.2 μm. When the average major axislength is less than 0.01 μmm, the metal powder is of superparamagnetism,so that the electromagnetic conversion characteristics markedlydeteriorate. On the other hand, when the average major axis lengthexceeds 0.4 μm, the metal particles have multiple magnetic domains, sothat the electromagnetic conversion characteristics deteriorate.Therefore, for securing the magnetic properties suitable for themultilayered magnetic recording medium, the magnetic powder ispreferably comprised of acicular fine particles having an average majoraxis length of 0.01 to 0.4 μm. The smaller the particle size of themagnetic powder, the poorer the dispersibility of the magnetic powder ina magnetic coating composition, and the recording wavelength for thelower layer is longer than that for the upper layer. Therefore, it isdesired that the particle size of the magnetic powder used in the lowerlayer is larger than that of the magnetic powder used in the upperlayer.

The metal powder preferably has a specific surface area (BET) of 25 to70 m²/g, more preferably 40 to 60 m²/g. When the specific surface areaof the metal powder is less than 25 m²/g, the compatibility of the metalpowder with a resin during the preparation of magnetic tape becomespoor, so that the electromagnetic conversion characteristicsdeteriorate. On the other hand, when the specific surface area exceeds70 m²/g, the dispersibility of the metal powder during the preparationof magnetic tape becomes poor, so that the electromagnetic conversioncharacteristics deteriorate.

The crystallite in the metal magnetic powder preferably has a size of 50to 250 Å, more preferably 100 to 200 Å. When the crystallite size isless than 50 Å, the magnetic powder is of superparamagnetism, so thatthe electromagnetic conversion characteristics markedly deteriorate. Onthe other hand, when the crystallite size exceeds 250 Å, noisesincrease, so that the electromagnetic conversion characteristicsdeteriorate. The crystallite in the metal magnetic powder is as follows.The metal magnetic powder is generally in a acicular form, but actuallyFe atoms in the metal magnetic powder are bonded together throughmetallic bounding into a lattice form. Therefore, the magnetic powder iscomprised of several to several tens thousand basic lattice-formcrystals, and thus the crystallite in the metal magnetic powder meansthe lattice-form crystal.

In the magnetic recording medium of the present invention, asingredients mixed into the magnetic layer other than the ferromagneticpowder, e.g., as a binder, an abrasive material, a flame retardant, ananticorrosive agent, a solvent used for preparation of the magneticcoating composition, and the nonmagnetic support, ones conventionallyknown can be used and there is no particular limitation.

As a material for the nonmagnetic support, ones generally used inmagnetic recording media can be used, and examples include polyester,such as polyethylene terephthalate and polyethylene naphthalate;polyolefin, such as polyethylene and polypropylene; cellulosederivatives, such as cellulose triacetate, cellulose diacetate, andcellulose acetate butyrate; vinyl resins, such as polyvinyl chloride andpolyvinylidene chloride; other plastics, such as polycarbonate,polyimide, and polyamideimide; metals, such as aluminum and copper;light alloys, such as aluminum alloys and titanium alloys; ceramic; andsingle crystalline silicon.

As a binder used in the magnetic layer, any materials conventionallyknown can be used. Examples include vinyl chloride-vinyl acetatecopolymers, vinyl chloride-vinyl acetate-vinyl alcohol copolymers, vinylchloride-vinylidene chloride copolymers, vinyl chloride-acrylonitrilecopolymers, vinyl chloride-vinyl acetate-maleic acid copolymers,acrylate-vinylidene chloride copolymers, acrylate-acrylonitrilecopolymers, methacrylic acid-vinylidene chloride copolymers,methacrylate-styrene copolymers, thermoplastic polyurethane resins,phenoxy resins, polyvinyl fluoride, vinylidene chloride-acrylonitrilecopolymers, butadiene-acrylonitrile copolymers,acrylonitrile-butadiene-methacrylic acid copolymers, polyvinyl butyral,cellulose derivatives, styrene-butadiene copolymers, polyester resins,phenolic resins, epoxy resins, thermosetting polyurethane resins, urearesins, melamine resins, alkyd resins, urea-formaldehyde resins,polyvinyl acetal resins, and mixtures thereof.

Especially desired are polyurethane resins, polyester resins, andacrylonitrile-butadiene copolymers, which are considered to impartflexibility; and cellulose derivatives, phenolic resins, and epoxyresins, which are considered to impart stiffness. These binders may bethose which are improved in durability by crosslinking an isocyanatecompound or those having an appropriate polar group introduced.

As the carbon black usable in the magnetic recording medium of thepresent invention, reference can be made to, for example, “Kabon-burakkuBinran (Carbon Black Handbook)” (edited by the Society of Carbon Black,published by TOSHO SHUPPAN-SHA CO., LTD. on May 25, 1971), and, withrespect to the type of carbon, there is no particular limitation.

As the carbon black used in the present invention, one having a DBP oilabsorption of 30 to 150 ml/100 g, preferably 50 to 150 to ml/100 g, anaverage particle size of 5 to 150 nm, preferably 15 to 50 nm, and aspecific surface area of 40 to 300 m²/g, preferably 100 to 250 m²/g, asmeasured by a BET method, is effective. Further, the carbon blackpreferably has a tap density of 0.1 to 1 g/cc and pH of 2.0 to 10. Whenthe carbon black has a larger DBP oil absorption, the viscosity of theresultant composition is too high, so that the dispersibility becomesmarkedly poor. On the other hand, when the carbon black has a smallerDBP oil absorption, the dispersibility is very poor and hence thedispersing step takes much time. When the carbon black has a smalleraverage particle size, the dispersion time is prolonged, but the surfaceproperties are excellent. When the carbon black has a larger averageparticle size, the surface properties are poor. Therefore, it ispreferred that the carbon black has an average particle size which fallswithin the above-mentioned range.

Examples of carbon black having the above-mentioned properties includetrade names RAVEN 1250, manufactured and sold by Columbia CarbonCorporation (particle size: 23 nm; BET value: 135.0 m²/g; DBP oilabsorption: 58.0 ml/100 g), RAVEN 1255 (particle size: 23 nm; BET value:125.0 m²/g; DBP oil absorption: 58.0 ml/100 g), RAVEN 1020 (particlesize: 27 nm; BET value: 95.0 m²/g; DBP oil absorption: 60.0 ml/100 g),RAVEN 1080 (particle size: 28 nm; BET value: 78.0 m²/g; DBP oilabsorption: 65.0 ml/100 g), RAVEN 1035, RAVEN 1040, RAVEN 1060, RAVEN3300, RAVEN 450, and RAVEN 780; and trade name SC, manufactured and soldby CONDUCTEX (particle size: 20 nm; BET value: 220.0 m²/g; DBP oilabsorption: 115.0 ml/100 g).

In addition, trade name #80, manufactured and sold by Asahi Carbon Co.,Ltd. (particle size: 23 nm; BET value: 117.0 m²/g; DBP oil absorption:113.0 ml/100 g); trade names #22B, manufactured and sold by MitsubishiChemical Industries Ltd. (particle size: 40 nm; BET value: 5.0 m²/g; DBPoil absorption: 131.0 ml/100 g), and #20B (particle size: 40 nm; BETvalue: 56.0 m²/g; DBP oil absorption: 115.0 ml/100 g); and trade namesBLACK PEARLS L, manufactured and sold by Cabot Specialty Chemicals Inc.(particle size: 24 nm; BET value: 250.0 m²/g; DBP oil absorption: 60.0ml/100 g), BLACK PEARLS 800 (particle size: 17.0 nm; BET value: 240.0m²/g; DBP oil absorption: 75.0 ml/100 g), BLACK PEARLS 1000, BLACKPEARLS 1100, BLACK PEARLS 700, and BLACK PEARLS 905, may be used.Further, as carbon having a larger particle size, MT CARBON(manufactured and sold by Columbia Carbon Corporation; particle size:350 nm), and Thermax MT can be used.

As the abrasive material, for example, α-alumina having an α rate of 90%or more, β-alumina, γ-alumina, silicon carbide, chromium oxide, ceriumoxide, α-iron oxide, corundum, silicon nitride, titanium carbide,titanium oxide, silicon dioxide, tin oxide, magnesium oxide, tungstenoxide, zirconium oxide, boron nitride, zinc oxide, calcium carbonate,calcium sulfate, barium sulfate, molybdenum disulfide, or acicularα-iron oxide obtained by dehydrating and annealing a material ofmagnetic iron oxide, and these materials, if desired, having a surfacetreated with aluminum and/or silica may be used individually or incombination.

The above nonmagnetic powder generally has a particle size in the rangeof 0.01 to 2 μm, preferably 0.015 to 1.00 μm, further preferably 0.015to 0.50 μm. If desired, the same effect can be obtained by usingnonmagnetic powder having different particle sizes in combination orusing solely nonmagnetic powder having a broad particle sizedistribution. The nonmagnetic powder generally has a tap density of 0.05to 2 g/cc, preferably 0.2 to 1.5 g/cc. The nonmagnetic powder generallyhas a specific surface area of 1 to 200 m²/g, desirably 5 to 100 m²/g,further desirably 7 to 80 m²/g. The nonmagnetic powder generally has acrystallite size in the range of 0.01 to 2 μm, preferably 0.015 to 1.00μm, further preferably 0.015 to 0.50 μm. The nonmagnetic powdergenerally has an oil absorption of 5 to 100 ml/100 g, desirably 10 to 80ml/100 g, further desirably 20 to 60 ml/100 g, as measured using DBP.The nonmagnetic powder generally has a specific gravity of 1 to 12,preferably 2 to 8. The nonmagnetic powder may be any forms of anacicular form, a sphere form, a cube form, and a plate form.

The nonmagnetic powder may not have a purity of 100% and, if desired,the nonmagnetic powder may have a surface treated with a compound. Inthis case, generally, the effect of the nonmagnetic powder is notsacrificed as long as the nonmagnetic powder has a purity of 70% ormore. For example, when titanium oxide is used as the nonmagneticpowder, one having a surface treated with alumina is generally used. Itis desired that the ignition loss is 20% or less. It is desired that theinorganic powder used in the present invention has a Mohs hardness of 6or more.

Further, as the abrasive material, conventionally known materials havinga Mohs hardness of 6 or more comprised mainly of, for example,α-alumina, β-alumina, fusion alumina, or titanium oxide may be usedindividually or in combination.

Specific examples of abrasive materials used in the present inventioninclude trade names UA5600 and UA5605, manufactured and sold by SHOWADENKO K.K.; trade names AKP-20, AKP-30, AKP-50, HIT-50, HIT-100, andZA-G1, manufactured and sold by Sumitomo Chemical Co., Ltd.; trade namesG5, G7, and S-1, manufactured and sold by Nippon Chemical IndustrialCO., LTD.; trade names TF-100, TF-120, TF-140, DPN 250BX, and DBN 270BX,manufactured and sold by TODA KOGYO CORP.; trade names TTO-51B, TTO-55A,TTO-55B, TTO-55C, TTO-55S, TTO-55D, FT-1000, FT-2000, FTL-100, FTL-200,M-1, S-1, and SN-100, manufactured and sold by Ishihara Sangyo KaishaLtd.; trade names ECT-52, STT-4D, STT-30D, STT-30, and STT-65C,manufactured and sold by TITAN KOGYO KABUSHIKI KAISHA; trade name T-1,manufactured and sold by MITSUBISHI MATERIALS CORPORATION; trade namesNS-O, NS-3Y, and NS-8Y, manufactured and sold by NIPPON SHOKUBAI CO.,LTD.; trade names MT-100S, MT-100T, MT-150W, MT-500B, MT-600B, andMT-100F, manufactured and sold by TAYCA CORPORATION; trade names FINEX-25, BF-1, BF-10, BF-20, BF-1L, and BF-10P, manufactured and sold bySAKAI CHEMICAL INDUSTRY CO., LTD.; trade names DEFIC-Y and DEFIC-R,manufactured and sold by Dowa Mining Co., Ltd.; and trade name Y-LOP,manufactured and sold by TITAN KOGYO KABUSHIKI KAISHA.

As the lubricant, any lubricants conventionally known can be used. Forexample, higher fatty acid ester, silicone oil, fatty acid-modifiedsilicone, fluorine-containing silicone and other fluorine lubricants,polyolefin, polyglycol, ester and metal salts of alkylphosphoric acid,polyphenyl ether, fluoroalkyl ether, amine lubricants, such as aminesalts of alkylcarboxylic acid and amine salts of fluoroalkylcarboxylicacid, alcohol having 12 to 24 carbon atoms (which may be unsaturated orbranched), and higher fatty acid having 12 to 24 carbon atoms can beused.

The higher fatty acid ester component may be higher fatty acid esterhaving 12 to 32 carbon atoms (which may be unsaturated or branched), andexamples include methyl esters, ethyl esters, propyl esters, isopropylesters, butyl esters, pentyl esters, hexyl esters, heptyl esters, andoctyl esters of lauric acid, myristic acid, palmitic acid, stearic acid,isostearic acid, arachic acid, oleic acid, eicosanoic acid, elaidicacid, behenic acid, linoleic acid, and linolenic acid.

Specific examples of compounds include butyl stearate, pentyl stearate,heptyl stearate, octyl stearate, isooctyl stearate, butoxyethylstearate, octyl myristate, isooctyl myristate, and butyl palmitate. Aplurality of lubricants may be mixed together.

As the flame retardant, in addition to the above-mentioned carbon black,conventionally known flame retardants including natural surfactants,nonionic surfactants, and cationic surfactants can be used.

In the present invention, coupling agents conventionally known can beused. Examples of coupling agents include silane coupling agents,titanate coupling agents, and aluminum coupling agents. The amount ofthe coupling agent added is preferably 0.05 to 10.00 parts by weight,more preferably 0.1 to 5.00 parts by weight, relative to 100 parts byweight of the magnetic powder.

Examples of silane coupling agents which can be preferably used includevinylsilane compounds, such as γ-methacryloxypropyltrimethoxysilane andvinyltriethoxysilane; epoxysilane compounds, such asβ-(3,4-epoxycyclohexyl)ethyltrimethoxysilane andγ-glycidoxypropyltrimethoxysilane; aminosilane compounds, such asγ-aminopropyltriethoxysilane and N-β(aminoethyl)γ-aminopropylmethyldimethoxysilane; and mercaptosilanecompounds, such as γ-mercaptopropyltrimethoxysilane.

Examples of titanate coupling agents include

-   tetra-n-butoxytitanium, tetraisopropoxytitanium,-   bis[2-{(2-aminoethyl)amino}ethanolate][2-{(2-aminoethyl)amino}ethanolate-0](2-propanolate)titanium,-   tris(isooctadecanoate-0)(2-propanolate)titanium,-   bis(ditridecylphosphite-0″)tetrakis(2-propanolate)dihydrozenetitanate,-   bis(dioctylphosphite-0″)tetrakis(2-propanolate)dihydrozenetitanate,-   tris(dioctylphosphite-0″)(2-propanolate)titanium,-   bis(dioctylphosphite-0″)[1,2-ethanediolate(2-)-0,0″]titanium,-   tris(dodecylbenzenesulfonate-0)(2-propanolate)titanium, and-   tetrakis[2,2-bis{(2-propenyloxy)methyl}-1-butanolate]titanate.

Specific examples of trade names of titanate coupling agents which canbe preferably used include PLENACT KR TTS, KR 46B, KR 55, KR 41B, KR38S, KR 138S, KR 238S, 338X, KR 12, KR 44, KR 9SA, and KR 34S,manufactured and sold by Ajinomoto-Fine-Techno Co., Inc.

Examples of aluminum coupling agents include acetoalkoxyaluminumdiisopropylate, and specific examples of trade names of aluminumcoupling agents which can be preferably used include PLENACT AL-M,manufactured and sold by Ajinomoto-Fine-Techno Co., Inc.

In the present invention, for imparting more excellent durability to themagnetic recording medium, an isocyanate curing agent having an averagefunctional group number of 2 or more may be added. Specifically, in thepresent invention, polymeric substances of polyisocyanate and adducts ofpolyisocyanate with polyol can be preferably used. Of these,isocyanurate having a cyclic skeleton, which is a trimer ofdiisocyanate, is a curing agent having remarkable reactivity andeffective to improve the durability.

Examples of isocyanate curing agents include aromatic polyisocyanate andaliphatic polyisocyanate, and preferred are adducts of thepolyisocyanate with an active hydrogen compound.

Examples of aromatic polyisocyanate include toluene diisocyanate (TDI),1,3-xylene diisocyanate, 1,4-xylene diisocyanate, 4,4′-diphenylmethanediisocyanate (MDI), p-phenyl diisocyanate, m-phenyl diisocyanate, and1,5-naphthyl diisocyanate.

Examples of aliphatic polyisocyanate include hexamethylene diisocyanate(HDI), dicyclohexylmethane diisocyanate, cyclohexane diisocyanate, andisophorone diisocyanate (IPDI).

Examples of active hydrogen compounds which form an adduct, togetherwith the above polyisocyanate, include ethylene glycol, 1,4-butanediol,1,3-butanediol, neopentyl glycol, diethylene glycol, trimethyrolpropane, and glycerol, and preferred are those having an averagemolecular weight of 100 to 5,000.

The amount of the curing agent added is generally 0 to 20 parts byweight, preferably 0 to 10 parts by weight, relative to the weight ofthe binder resin. Theoretically, the weight of the curing agentcontaining isocyanate in an amount corresponding to the equivalentamount of the active hydrogen contained in the polyurethane resincomposition (or binder resin composition) is satisfactory. However, inthe actual production, isocyanate in the curing agent is lost due to thereaction with water, and hence isocyanate in an amount corresponding tothe equivalent amount of the active hydrogen is usually unsatisfactory.For this reason, it is effective to add an excess curing agent in anamount 10 to 50% larger than the equivalent amount of the activehydrogen.

When a curing agent comprised of polyisocyanate is used in the magneticcoating composition, the magnetic coating composition is applied, andthen a curing reaction is promoted at 40 to 80° C. for several hours,thus obtaining larger binding strength.

Examples of solvents which can be used for preparation of the magneticcoating composition include ketone solvents, such as acetone, methylethyl ketone, methyl isobutyl ketone, and cyclohexanone; ester solvents,such as methyl acetate, ethyl acetate, butyl acetate, ethyl lactate, andethyl acetate monoethyl ether; glycol ether solvents, such as glycolmonoethyl ether and dioxane; aromatic hydrocarbon solvents, such asbenzene, toluene, and xylene; and chlorine-containing solvents, such asmethylene chloride, ethylene chloride, carbon tetrachloride, chloroform,ethylene chlorohydrin, and dichlorobenzene. Other organic solventsconventionally known can be used.

As a method for preparing the magnetic coating composition, any methodsconventionally known can be used. For example, a roll mill, a ball mill,a sand mill, a trone mill, a high-speed stone mill, a basket mill, adisper, a homomixer, a kneader, a continuous kneader, an extruder, ahomogenizer, and an ultrasonic dispersing machine can be used.

In the magnetic recording medium of the present invention, a nonmagneticback coat layer may be formed on another surface of the nonmagneticsupport that is not the surface on which the magnetic layers are formed.The back coat layer may have a thickness of 0.3 to 1.0 μm, andconventionally known materials for the back coat layer can be used.

Prior to application of the magnetic coating composition directly ontothe nonmagnetic support, an undercoat layer, such as an adhesive layer,may be applied to the nonmagnetic support, or the nonmagnetic supportmay be subjected to pretreatment, such as a corona discharge treatmentor an electron beam radiation treatment.

Examples of methods for applying the magnetic coating composition to thenonmagnetic support include air doctor coating, blade coating, rodcoating, extrusion coating, air knife coating, squeeze coating,impregnation coating, reverse roll coating, gravure coating, transferroll coating, and cast coating. Other methods can be used, and furtherco-extrusion multilayer coating may be used.

If desired, for improving the binding strength and the like, a layer(undercoat layer) comprised mainly of the binder conventionally knownmay be formed between the nonmagnetic support and the first magneticlayer (lower layer).

EXAMPLES

Hereinbelow, the present invention will be described in more detail withreference to the following Examples, which should not be construed aslimiting the scope of the present invention.

Formation of Magnetic Layer:

-   -   Coating compositions having the formulations shown below for        individual magnetic layers were individually prepared.

Preparation of Magnetic Coating Composition for Upper Layer:

-   -   Metal magnetic powder: 100 Parts by weight    -   Average major axis length: 0.01 to 0.5 μm    -   Specific surface area: 50 to 70 m²/g as measured by BET method    -   Crystallite size: 50 to 250 Å    -   Coercive force Hc: 100 to 200 (kA/m)    -   Saturation magnetization σs: 100 to 200 (Am²/kg)    -   {Details are shown in FIGS. 15 to 18 (Tables 1 to 4).}    -   Vinyl chloride copolymer: 15 Parts by weight    -   (trade name: MR-110; manufactured and sold by Nippon Zeon Co.,        Ltd.)    -   Polyester polyurethane resin: 5 Parts by weight    -   (isophthalic acid/terephthalic acid/butanediol-MDI polyurethane;        molecular weight: 25,000; polar group SO₃Na content: 0.2 wt %)    -   α-Al₂O₃: 10 Parts by weight    -   (trade name: HIT-5010; manufactured and sold by Sumitomo        Chemical Co., Ltd.)    -   Carbon black: 1 Part by weight    -   (trade name: BP-L; manufactured and sold by Cabot Specialty        Chemicals Inc.)    -   Polyisocyanate: 4 Parts by weight    -   (trade name: CORONATE L; manufactured and sold by NIPPON        POLYURETHANE INDUSTRY CO., LTD.; Polyisocyanate was added        immediately before coating.)    -   Myristic acid: 1 Part by weight    -   Butyl stearate: 1 Part by weight    -   Methyl ethyl ketone: 80 Parts by weight    -   Methyl isobutyl ketone: 80 Parts by weight    -   Toluene: 80 Parts by weight

A magnetic mixture having the above formulation for upper layer coatingcomposition was kneaded by means of a three-roll, and then dispersedusing a sand mill, and 4 parts by weight of polyisocyanate and 1 part byweight of myristic acid were added to the resultant dispersion andfiltered by means of a filter having an average pore diameter of 1 μm toprepare a magnetic coating composition for upper layer.

Preparation of Magnetic Coating Composition for Lower Layer:

-   -   Metal magnetic powder: 100 Parts by weight    -   Average major axis length: 0.01 to 0.5 μm    -   Specific surface area: 25 to 60 m²/g as measured by BET method    -   Crystallite size: 50 to 250 Å    -   Coercive force Hc: 90 to 180 (kA/m)    -   Saturation magnetization σs: 80 to 150 (Am²/kg)    -   {Details are shown in FIGS. 15 to 18 (Tables 1 to 4).}    -   Vinyl chloride copolymer: 15 Parts by weight    -   (tradename: MR-110; manufactured and sold by Nippon Zeon Co.,        Ltd.)    -   Polyester polyurethane resin: 5 Parts by weight    -   (isophthalic acid/terephthalic acid/butanediol-MDI polyurethane;        molecular weight: 25,000; polar group SO₃Na content: 0.2 wt %)    -   Carbon black: 5 Parts by weight    -   (trade name: BP-L; manufactured and sold by Cabot Specialty        Chemicals Inc.)    -   Polyisocyanate: 4 Parts by weight    -   (trade name: CORONATE L; manufactured and sold by NIPPON        POLYURETHANE INDUSTRY CO., LTD.; Polyisocyanate was added        immediately before coating.)    -   Myristic acid: 1 Part by weight    -   Butyl stearate: 1 Part by weight    -   Methyl ethyl ketone: 80 Parts by weight    -   Methyl isobutyl ketone: 80 Parts by weight    -   Toluene: 80 Parts by weight

A magnetic mixture having the above formulation for lower layer coatingcomposition was kneaded by means of a continuous kneader, and thendispersed using a sand mill, and 4 parts by weight of polyisocyanate and1 part by weight of myristic acid were added to the resultant dispersionand filtered by means of a filter having an average pore diameter of 1μm to prepare a magnetic coating composition for lower layer.

A back coat coating composition having the formulation shown below wasprepared.

Preparation of Nonmagnetic Coating Composition for Back Coat:

-   -   Carbon black: 100 Parts by weight    -   (average particle size: 20 nm)    -   Carbon black: 5 Parts by weight    -   (average particle size: 350 nm)    -   Polyurethane resin: 25 Parts by weight    -   (polycarbonate polyol/neopentyl glycol HDI polyurethane;        molecular weight: 35,000; N-methyldiethanolamine content: 0.2 wt        %)    -   Nitrocellulose: 15 Parts by weight    -   (tradename: NC-1/2H; manufactured and sold by Asahi Kasei        Corporation)    -   Polyisocyanate: 20 Parts by weight    -   (trade name: CORONATE L; manufactured and sold by NIPPON        POLYURETHANE INDUSTRY CO., LTD.; Polyisocyanate was added        immediately before coating.)    -   Methyl ethyl ketone: 180 Parts by weight    -   Methyl isobutyl ketone: 180 Parts by weight    -   Toluene: 180 Parts by weight

A mixture having the above formulation for nonmagnetic coatingcomposition was kneaded by means of a three-roll, and then dispersedusing a sand mill, and 20 parts by weight of polyisocyanate was added tothe resultant dispersion and filtered by means of a filter having anaverage pore diameter of 1 μm to prepare a nonmagnetic coatingcomposition for back coat.

The previously prepared magnetic coating compositions were applied to apolyethylene terephthalate film having a thickness of 10 μm so that thethickness of the resultant individual magnetic layers became 4.0 μm toform two magnetic layers simultaneously, and then dried and calendered,followed by curing. Then, the above-prepared nonmagnetic coatingcomposition for back coat was applied to another surface of thepolyethylene terephthalate film, that is not the surface on which themagnetic layers were formed, so that the thickness of the resultant backcoat layer became 0.8 μm, and then dried. The resultant wide magneticfilm was cut into ½ inch in width to form a video tape. Further, thevideo tapes formed were individually incorporated into HDCAM cassettes,manufactured and sold by Sony Corporation, to obtain cassette tapes inExamples 1 to 8 and Comparative Examples 1 to 14.

Measurement Method:

(Measurement of Electromagnetic Conversion Characteristics)

-   -   With respect to each of the samples incorporated into HDCAM        cassettes, an output was individually measured using a digital        video signal at 46.98 MHz and analog audio signals at 1 kHz and        12 kHz by means of HDCAM video recorder (HDW-500), manufactured        and sold by Sony Corporation, taking as 0 dB the output of HDCAM        video tape (BCT-124HDL), manufactured and sold by Sony        Corporation. (The output was measured in the same manner.) In        FIGS. 17 and 18 (Tables 3 and 4), the outputs in Example 6 and        Example 8, respectively, were taken as 0 dB. Further, the analog        audio frequency characteristic (F characteristic) was determined        by subtracting the output at 12 kHz from the output at 1 kHz.        (The F characteristic was determined in the same manner.)

In the measurement of the electromagnetic conversion characteristics, itis considered that a sample having an output lower than the output ofthe control tape by 0.5 dB or more has poor properties, and that asample having an output lower than the output of the control tape by 1.0dB or more cannot meet the standards in various formats. In themeasurement of the analog audio frequency characteristic, it isconsidered that a sample having a measurement value of −1.5 to 1.5 dB isexcellent and has no problem for use.

(Measurement of Durability)

-   -   100-Hour recording and reproducing were conducted by means of        HDCAM video recorder (HDW-500), manufactured and sold by Sony        Corporation, to measure the output waveform of the video        signals, and the durability was evaluated in accordance with the        following criteria:    -   ◯: No deterioration occurred in the output of the video signals.    -   Δ: The output deteriorated and then recovered, or the lowering        of the output was 2.0 dB or less.    -   X: Head clogging occurred.

(Measurement of Weathering Resistance)

-   -   The tapes in the Examples and Comparative Examples were        individually stored in an environment at a temperature of 45° C.        at a humidity of 80% for one month, and reduction ΔΦ of the        residual magnetic flux of each tape between before and after the        storage was measured, and the weathering resistance was        evaluated in accordance with the following criteria:    -   ◯: ΔΦ was −5% or less.    -   Δ: ΔΦ was −5 to −10%.    -   X: ΔΦ was −10% or more.

The results are shown in FIGS. 19 to 22 (Tables 5 to 8).

As can be seen from FIG. 19 (Table 5), in Comparative Example 1 in whichthe lower layer having a thickness of 3 μm was solely formed as amagnetic layer, the output of digital signals was very low. In addition,the Co content of the magnetic layer was too small, and hence both thedurability and the weathering resistance were poor.

Further, in Comparative Example 2 in which the upper layer having athickness of 3 μm was solely formed as a magnetic layer, the output ofanalog audio signals was very low.

In Examples 1 to 3 and Comparative Examples 3 to 6 in which the upperlayer and lower layer had different thickness, when the thickness of theupper layer was less than 0.01 μm (Comparative Examples 3 and 4), thecoating layer had a problem of discontinuity (a defect was caused in thecoating layer), so that the output of digital signals was an unstableand considerably low value. In addition, in Comparative Examples 5 and 6in which the thickness of the upper layer exceeded 0.5 μm, the digitaloutput was likely to be lowered due to the thickness demagnetization.Further, when the thickness of the lower layer was less than 1.0 μm(Comparative Examples 3 and 5), satisfactory analog audio propertiescould not be obtained.

FIG. 4 is a graph showing the electromagnetic conversion characteristicswhen, as shown in FIG. 23 (Table 9), the thickness of the upper layerwas fixed at 0.3 μm, the σs(B) and Hc(B) of the upper layer magneticpowder were fixed at 135 Am²/kg and 135 kA/m, respectively, the Cocontent of the upper layer magnetic powder was fixed at 10 at. %, theσs(A) and Hc(A) of the lower layer magnetic powder were fixed at 120Am²/kg and 115 kA/m, respectively, the Co content of the lower layermagnetic powder was fixed at 3 at. %, and only the thickness of thelower layer was changed. The results are shown also in FIG. 24 (Table10).

As can be seen from FIG. 4 and FIG. 24 (Table 10), when the thickness ofthe lower layer is less than 1 μm, satisfactory analog audioelectromagnetic conversion characteristics cannot be obtained due to thetoo small thickness. Further, it is apparent that, when the thickness ofthe lower layer exceeds 4 μm, the electromagnetic conversioncharacteristics are saturated and self demagnetization occurs due to thetoo large thickness of the magnetic layer. Thus, when the thickness ofthe lower layer falls in a specific range, i.e., the range of from 1 to4 μm, the electromagnetic conversion characteristics of analog signalscan be improved.

FIG. 5 is a graph showing the electromagnetic conversion characteristicswhen, as shown in FIG. 25 (Table 11), the thickness of the lower layerwas fixed at 2.7 μm, the σs(B) and Hc(B) of the upper layer magneticpowder were fixed at 135 Am²/kg and 135 kA/m, respectively, the Cocontent of the upper layer magnetic powder was fixed at 10 at. %, theσs(A) and Hc(A) of the lower layer magnetic powder were fixed at 120Am²/kg and 115 kA/m, respectively, the Co content of the lower layermagnetic powder was fixed at 3 at. %, and only the thickness of theupper layer was changed. The results are shown also in FIG. 26 (Table12).

As can be seen from FIG. 5 and FIG. 26 (Table 12), when the thickness ofthe upper layer is less than 0.01 μm, the coating layer had a problem ofdiscontinuity (a defect was caused in the coating layer), so that theoutput of digital signals is an unstable and considerably low value.Further, it is apparent that, when the thickness of the upper layerexceeds 0.5 μm, the digital output is likely to be lowered due to thethickness demagnetization. Thus, when the thickness of the upper layerfalls in a specific range, i.e., the range of from 0.01 to 0.5 μm, theelectromagnetic conversion characteristics of digital signals can beimproved.

As can be seen from FIG. 20 (Table 6), in Example 4 in which therelationship: Hc(A)/Hc(B)≦1.0 was satisfied, excellent measurementvalues could be obtained in respect of all the video and analog audiooutputs and the frequency characteristic. However, in Example 4 in whichthe relationship: Hc(A)/Hc(B)≧0.9 was satisfied, the output of digitalsignals was slightly low due to the lower layer having the aboverelationship, as compared to the output in Example 2. Thus, it isimportant that the upper and lower layers satisfy the relationship:Hc(A)/Hc(B)≦1.0, especially, more preferably the relationship:Hc(A)/Hc(B)≦0.9.

By contrast, in Comparative Example 7 in which the σs(A) of the lowerlayer was higher than the σs(B) of the upper layer and the relationship:σs(A)/σs(B)≦1.0 was not satisfied, the output was low, as compared tothe output in Example 2. Thus, with respect to the saturationmagnetization σs, it is important that the upper layer (B) and the lowerlayer (A) satisfy the relationship: σs(A)/σs(B)≦1.0.

FIG. 21 (Table 7) shows the results obtained when the upper layer hadσs(B) and Hc(B) upper limits of 200 Am²/kg and 200 kA/m, respectively,and this upper layer was combined with various lower layers.

As can be seen from FIG. 21 (Table 7), in Comparative Example 8 in whichthe σs of the lower layer was less than 80 Am²/kg and the Hc of thelower layer was less than 90 kA/m, the analog audio output was low, ascompared to the output in Example 6. Further, in Comparative Example 9in which the σs of the lower layer exceeded 150 Am²/kg and the Hcexceeded 180 kA/m, the analog audio output was low like in ComparativeExample 8.

In Comparative Example 10, the lower layer had σs(A) and Hc(A) lowerlimits of 80 Am²/kg and 90 kA/m, respectively, the upper layer had alarge Co content (higher than 50 at. %), the σs(B) of the upper layerexceeded 200 Am²/kg, and the Hc(B) of the upper layer exceeded 50 kA/m.Further, in Comparative Example 11, the lower layer had σs(A) and Hc(A)upper limits of 150 Am²/kg and 180 kA/m, respectively, the upper layerhad a large Co content (higher than 50 at. %), the σs(B) of the upperlayer exceeded 200 Am²/kg, and the Hc(B) of the upper layer exceeded 50kA/m.

In Comparative Examples 10 and 11, the upper layer had σs and Hc whichexceeded the respective upper limits defined in the present invention.Therefore, the effect of the cohesion of the magnetic powder was toolarge, and hence the analog audio output was low, as compared to theoutput in Examples 5 and 7.

FIG. 22 (Table 8) shows the results obtained when the upper layer hadσs(B) and Hc(B) lower limits of 100 Am²/kg or less and 100 kA/m or less,respectively, and this upper layer was combined with various lowerlayers.

In Comparative Examples 12 and 13, the lower layer had σs(A) and Hc(A)which fell in the respective ranges defined in the present invention,but the σs(B) and Hc(B) of the upper layer were too small, the output ofdigital signals was very low. In addition, in Comparative Example 14 inwhich the σs of the lower layer was also too small, a high analog audiooutput could not be obtained.

FIG. 6 is a graph showing the electromagnetic conversion characteristicswhen, as shown in FIG. 27 (Table 13), the Hc(A) of the lower layer wasfixed at 115 kA/m, the σs(B) and Hc(B) of the upper layer were fixed at135 Am²/kg and 135 kA/m, respectively, the thickness of the lower layerwas fixed at 2.7 μm, the thickness of the upper layer was fixed at 0.3μm, and the σs(A) of the lower layer was changed. The results are shownalso in FIG. 28 (Table 14).

As can be seen from FIG. 6 and FIG. 28 (Table 14), when the σs(A) of thelower layer was less than 80 Am²/kg, excellent electromagneticconversion characteristics of analog signals could not be secured.Further, when the σs(A) of the lower layer exceeded 150 Am²/kg, theeffect of the demagnetization was large, so that the electromagneticconversion characteristics of analog signals deteriorated.

As is apparent from the above, it is important that the σs(A) of thelower layer is 80 to 150 Am²/kg. When the σs(A) of the lower layer fallsin this range, the electromagnetic conversion characteristics of analogsignals can be improved.

FIG. 7 is a graph showing the electromagnetic conversion characteristicswhen, as shown in FIG. 29 (Table 15), the σs(A) of the lower layer wasfixed at 120 Am²/kg, the σs(B) and Hc(B) of the upper layer were fixedat 135 Am²/kg and 135 kA/m, respectively, the thickness of the lowerlayer was fixed at 2.7 μm, the thickness of the upper layer was fixed at0.3 μm, and the Hc(A) of the lower layer was changed. The results areshown also in FIG. 30 (Table 16).

As can be seen from FIG. 7 and FIG. 30 (Table 16), when the Hc(A) of thelower layer is less than 90 kA/m, the output of digital signals islowered. In addition, the above Hc(A) is too small for the lower layermagnetic powder having a σs of 120 Am²/kg, and therefore the magneticpowder has markedly poor stability and hence has poor weatheringresistance. Further, when the Hc(A) of the lower layer is 110 kA/m,namely, less than 114.75 kA/m {less than 85% of the Hc(B) of the upperlayer}, the output at a frequency as low as 1 kHz is increased, but theoutput at a frequency as high as 12 kHz is lowered, and the frequencycharacteristic tends to deteriorate. From the above, it is found thatthe more preferred range of the Hc(A) of the lower layer for thefrequency characteristic is 85 to 100%, further preferably 85 to 90% ofthe Hc(B) of the upper layer.

As is apparent from the above, it is important that the Hc(A) of thelower layer is 90 to 180 kA/m and that the upper and lower layerssatisfy the relationship: Hc(A)/Hc(B)≦1.0, more preferably0.85≦Hc(A)/Hc(B)≦1.0, further preferably 0.85≦Hc(A)/Hc(B)≦0.9. When theHc(A) and Hc(B) fall in the above range, the electromagnetic conversioncharacteristics of analog signals can be improved.

FIG. 8 is a graph showing the electromagnetic conversion characteristicswhen, as shown in FIG. 31 (Table 17), the σs(A) and Hc(A) of the lowerlayer were fixed at 120 Am²/kg and 115 kA/m, respectively, the Hc(B) ofthe upper layer was fixed at 135 kA/m, the thickness of the lower layerwas fixed at 2.7 μm, the thickness of the upper layer was fixed at 0.3μm, and the σs(B) of the upper layer was changed. The results are shownalso in FIG. 32 (Table 18).

As can be seen from FIG. 8 and FIG. 32 (Table 18), when the σs(B) of theupper layer is lower than the σs(A) of the lower layer, the output ofdigital signals becomes markedly poor. Further, it is apparent that,when the σs(B) of the upper layer exceeds 200 Am²/kg, the dispersibilityin the magnetic coating composition deteriorates and the composition isunsuitable for formation of a thin film, so that both the digital outputand the analog output are lowered.

As is apparent from the above, it is important that the σs(B) of theupper layer is 100 to 200 Am²/kg, especially preferably equal to orhigher than the σs(A) of the lower layer. When the σs(B) falls in thisrange, the electromagnetic conversion characteristics of digital signalscan be improved.

FIG. 9 is a graph showing the electromagnetic conversion characteristicswhen, as shown in FIG. 33 (Table 19), the σs(A) and Hc(A) of the lowerlayer were fixed at 120 Am²/kg and 115 kA/m, respectively, the σs(B) ofthe upper layer was fixed at 135 Am²/kg, the thickness of the lowerlayer was fixed at 2.7 μm, the thickness of the upper layer was fixed at0.3 μm, and the Hc(B) of the upper layer was changed. The results areshown also in FIG. 34 (Table 20).

As can be seen from FIG. 9 and FIG. 34 (Table 20), when the Hc(B) of theupper layer exceeded 200 kA/m, recording on the upper layer wasdifficult due to the upper layer having the high Hc(B), and hence thedigital output was lowered. Further, when the Hc(B) of the upper layerand the Hc(A) of the lower layer satisfied the relationship:Hc(A)/Hc(B)>1.0, recording on the upper layer was inhibited due to thelower layer having the above Hc(A), and hence the digital output wasmarkedly lowered.

As is apparent from the above, it is important that the Hc(B) of theupper layer is 100 to 200 kA/m and that the upper and lower layerssatisfy the relationship: Hc(A)/Hc(B)≦1.0. When the Hc(A) and Hc(B) fallin the above range, the electromagnetic conversion characteristics ofdigital signals can be improved.

FIG. 10 is a graph showing the electromagnetic conversioncharacteristics when, as shown in FIG. 35 (Table 21), the σs(B) of theupper layer was fixed at 135 Am²/kg, the Hc(A) of the lower layer wasfixed at 115 kA/m, the Hc(B) of the upper layer was fixed at 135 kA/m,the thickness of the lower layer was fixed at 2.7 μm, the thickness ofthe upper layer was fixed at 0.3 μm, and the σs(A)/σs(B) ratio waschanged. The results are shown also in FIG. 36 (Table 22).

FIG. 11 is a graph showing the electromagnetic conversioncharacteristics when, as shown in FIG. 37 (Table 23), the σs(A) andHc(A) of the lower layer were fixed at 120 Am²/kg and 115 kA/m,respectively, the Hc(B) of the upper layer was fixed at 135 kA/m, thethickness of the lower layer was fixed at 2.7 μm, the thickness of theupper layer was fixed at 0.3 μm, and the σs(A)/σs(B) ratio was changed.The results are shown also in FIG. 38 (Table 24).

As can be seen from FIGS. 10 and 11, and FIGS. 36 and 38 (Tables 22 and24), when the σs(A)/σs(B) ratio exceeded 1, the output of digitalsignals was lowered.

As is apparent from the above, it is important that the σs(A)/σs(B)ratio is 1 or less. When the σs(A)/σs(B) ratio falls in this range, theelectromagnetic conversion characteristics of digital signals can beimproved.

FIG. 12 is a graph showing the electromagnetic conversioncharacteristics when, as shown in FIG. 39 (Table 25), the Hc(B) of theupper layer was fixed at 135 kA/m, the σs(A) of the lower layer wasfixed at 120 Am²/kg, the σs(B) of the upper layer was fixed at 135Am²/kg, the thickness of the lower layer was fixed at 2.7 μm, thethickness of the upper layer was fixed at 0.3 μm, and the Hc(A)/Hc(B)ratio was changed. The results are shown also in FIG. 40 (Table 26).

FIG. 13 is a graph showing the electromagnetic conversioncharacteristics when, as shown in FIG. 41 (Table 27), the Hc(A) andσs(A) of the lower layer were fixed at 115 kA/m and 120 Am²/kg,respectively, the σs(B) of the upper layer was fixed at 135 Am²/kg, thethickness of the lower layer was fixed at 2.7 μm, the thickness of theupper layer was fixed at 0.3 μm, and the Hc(A)/Hc(B) ratio was changed.The results are shown also in FIG. 42 (Table 28).

As can be seen from FIGS. 12 and 13, and FIGS. 40 and 42 (Tables 26 and28), when the Hc(A)/Hc(B) ratio exceeds 1.0, the output of analogsignals especially at a frequency as low as 1 kHz is lowered. Further,it is apparent that, when the Hc(A)/Hc(B) ratio is less than 0.85, theanalog audio frequency characteristic tends to be lowered.

As is apparent from the above, it is important that the Hc(A)/Hc(B)ratio is 1.0 or less, more preferably 0.85 to 0.9. When the Hc(A)/Hc(B)ratio falls in this range, the electromagnetic conversioncharacteristics of analog signals and digital signals can be improved.

FIG. 14 is a graph showing the electromagnetic conversioncharacteristics when, as shown in FIG. 43 (Table 29), the Hc(B) of theupper layer was fixed at 130 kA/m, the σs(A) of the lower layer wasfixed at 110 Am²/kg, the σs(B) of the upper layer was fixed at 120Am²/kg, the thickness of the lower layer was fixed at 2.5 μm, thethickness of the upper layer was fixed at 0.5 μm, and the Hc(A)/Hc(B)ratio was changed. The results are shown also in FIG. 44 (Table 30).

As can be seen from FIG. 14 and FIG. 44 (Table 30), when the Hc(A)/Hc(B)ratio is 1.0 or less, the video and analog audio properties areexcellent. Especially when the Hc(A)/Hc(B) ratio is 0.85 to 1.0, morepreferably 0.85 to 0.9, the balance between the video and analog audioproperties is excellent.

1. A magnetic recording medium comprising a nonmagnetic support, a firstmagnetic layer comprised of magnetic powder (A), and a second magneticlayer comprised of magnetic powder (B), wherein said first and secondmagnetic layers are formed on said nonmagnetic support in this order,said magnetic powder (A) used in said first magnetic layer having thefollowing characteristics: a saturation magnetization ss(A) of 80 to 150Am2/kg; and a coercive force Hc(A) of 90 to 180 kA/m, said magneticpowder (B) used in said second magnetic layer having the followingcharacteristics: a saturation magnetization ss(B) of 100 to 200 Am2/kg;and a coercive force Hc(B) of 100 to 200 kA/m, wherein said first andsecond magnetic layers satisfy the following relationships:ss(A)/ss(B)≦1.0, and Hc(A)/Hc(B)≦1.0, wherein said first magnetic layerhas a thickness T(A) in the range represented by the following formula:1.0 μm≦T(A)≦4.0 μm, and wherein said second magnetic layer has athickness T(B) in the range represented by the following formula:0.01 μm≦T(B)≦0.5 μm, and wherein said magnetic powder (A) used in saidfirst magnetic layer is comprised of acicular iron and has a cobaltcontent of 0 to 5 at. %, and said magnetic powder (B) used in saidsecond magnetic layer is comprised of acicular iron and has a cobaltcontent of 5 to 50 at. % and further wherein said second magnetic layerhas a thickness that is greater than 0.01 micrometer.